The present invention relates generally to continuous emissions monitoring of exhaust flue gas streams. More specifically, the present invention relates to the humidification of calibration checking in continuous emissions monitoring systems.
The United States Environmental Protection Agency (EPA) identifies sources of mercury (Hg) emissions in the U.S. to be utility boilers, waste incinerators that burn mercury-containing wastes (municipal and medical), coal-fired industrial boilers and cement kilns that burn coal-based fuels. A particularly significant source of mercury emissions is coal-fired power plants.
To quantify the emissions from a particular source, a continuous emissions monitoring system (CEMS) is employed for mercury. There are three forms of mercury in exhaust flue gas stream of a coal-fired power plant that may be monitored by a CEMS. These forms are gaseous elemental mercury, gaseous oxidized mercury and particulate bound mercury that is either elemental or oxidized.
Mercury in the gaseous forms is relatively sticky and has a strong affinity to attach to a wide variety of interior surfaces of CEMS components. Such gaseous mercury is extremely difficult to handle and transport through an extractive gas sampling system to a gas analyzer for measurement. Exhaust flue gases usually contain relatively low concentrations of gaseous mercury that must be detected and the sticky gaseous mercury present readily attaches to surfaces of the components of the CEMS. This renders any measurement made on the sample not truly representative of what is conducted in the exhaust stack.
Particulates and other undesirable material from the stack gas sample might also adhere to surfaces of the CEMS components due to moisture present in exhaust flue gas. This causes the adsorption of elemental mercury onto particles adhered to the wetted surfaces.
The EPA has mandated restrictive controls on mercury emissions. A total mercury measurement is required for regulatory monitoring and the evaluation of mercury control technologies and manufacturing processes requires accurate measurements of gaseous mercury. One example is that the EPA requires a “span gas check” accuracy of plus or minus ten percent (±10%) of a sample range. Accordingly, there exists a need for the development of a reliable and accurate technology capable of verifying the measurement of mercury emitted in an exhaust flue gas stream.